Doppler Compensation and Beamforming for High Mobility OFDM Transmissions in Multipath

The paper focuses on the use of receive beamforming (BF) for high speed train (HST) scenario under independent Doppler for the different multipath components in a Rician Fading environment. To combat ICI, we not only null out the ICI in the frequency domain, but do a pre-processing in the time domain via frequency correction (demodulation) to maximise the signal part at the output of the FFT. To obtain a suitable demodulation frequency, location aware and location agnostic approaches are considered. Cyclic prefix (CP) based estimation method is also considered as part of location information agnostic approach. In the location-aware approach, a technique that uses both the LoS and dominant scatterer information is also proposed. The paper then provides the optimal weights for the maximisation of the SINR criterion from a theoretical and practical perspective. In the case of linear approximation of the channel variation, the ICI is shown to be a rank 1 interferer and hence can be nulled out with just 2 receiver antennas. Finally, all the methods are compared via simulations. We conclude that in an LTE OFDM system simple, low complex, location agnostic BF schemes are very effective against ICI even with just two receive antennas.